The present invention relates generally to the field of metallurgy and to a bearing component such as a rolling element or ring formed from a bearing steel. A heat- treatment induces a compressive residual stress (CRS) in a surface region of the bearing steel with the corollary of an improvement in mechanical properties, for example rolling contact fatigue performance.
Bearings are devices that permit constrained relative motion between two parts. Rolling element bearings comprise inner and outer raceways and a plurality of rolling elements (ball bearings or roller bearings). For long-term reliability and performance it is important that the various elements have a high resistance to rolling fatigue, wear and creep.
Conventional techniques for manufacturing metal components involve hot-rolling or hot-forging to form a bar, rod, tube or ring, followed by a soft forming process to obtain the desired component. Surface hardening processes are well known and are used to locally increase the hardness of surfaces of finished components so as to improve, for example, wear resistance and fatigue resistance.
A number of surface hardening processes are known for improving rolling contact fatigue performance. Shot peening involves bombarding the surface of the metal component with rounded shot to locally harden surface layers. However, this process results in a rough surface finish which can create other problems and therefore additional steps need to be taken to improve the surface finish. This adds to productions costs.
Case-hardening may also be achieved by heating the steel component in a carbonaceous medium to increase the carbon content, followed by quenching and tempering. This thermochemcial process is known as carburizing and results in a surface chemistry that is quite different from that of the core of the component. Alternatively, the hard surface layer may be formed by rapidly heating the surface of a medium/high carbon steel to above the ferrite/austenite transformation temperature, followed by quenching and tempering to result in a hard surface layer. Heating of the surface has traditionally been achieved by flame hardening, although laser surface-hardening and induction hardening are now often used. Induction hardening involves heating the steel component by exposing it to an alternating magnetic field to a temperature within or above the transformation range, followed by quenching. Heating occurs primarily in the surface of the component, with the core of the component remaining essentially unaffected. The penetration of the field is inversely proportional to the frequency of the field and thus the depth of the hardening can be adjusted in a simple manner. The penetration of the field also depends on the power density and interaction time.
An alternative to case-hardening is through-hardening. Through-hardened components differ from case-hardened components in that the hardness is uniform or substantially uniform throughout the component. Through-hardened components are also generally cheaper to manufacture than case-hardened components because they avoid the complex heat-treatments associated with carburizing, for example.
The steel grades that are used depend on the component section thickness. For components having a wall thickness of up to about 20 mm, DIN 100Cr6 is typically used. For larger section sizes, higher alloyed grades are used such as for example, DIN 100CrMo7-3, DIN 100CrMnMo7, DIN 100CrMo7-4, or DIN 100CrMnMo8.
For through-hardened bearing steel components, two heat-treating methods are available: martensite hardening or austempering. Component properties such as toughness, hardness, microstructure, retained austenite content, and dimensional stability are associated with or affected by the particular type of heat treatment employed.
The martensite through-hardening process involves austenitising the steel prior to quenching below the martensite start temperature. The steel may then be low-temperature tempered to stabilize the microstructure. The martensite through-hardening process typically results in a compressive residual stress (CRS) of from 0 to +100 MPa between the WCS (working contact surface) and down to an approximately 1.5 mm depth below the WCS.
The bainite through-hardening process involves austenitising the steel prior to quenching above the martensite start temperature. Following quenching, an isothermal bainite transformation is performed. Bainite through-hardening is sometimes preferred in steels instead of martensite through-hardening. This is because a bainitic structure may possess superior mechanical properties, for example toughness and crack propagation resistance. The bainite though-hardening process results in a CRS of from 0 to −100 MPa between the WCS and down to an approximately 1.5 mm depth below the WCS.
Numerous conventional heat-treatments are known for achieving martensite through-hardening and bainite through-hardening.
U.S. Pat. No. 5,853,660 relates to a bearing steel consisting of 0.70 to 0.93 wt % of C, 0.15 to 0.50 wt % of Si, 0.50 to 1.10 wt % of Mn and 0.3 to 0.65 wt % of Cr, and the balance Fe, wherein the ratio of Cr to C is 0.4 to 0.7.
U.S. Pat. No. 4,023,988 relates to methods of improving the rolling contact fatigue life for metallic bearing components, particularly chromium steel components designated commercially as SAE 52100 (carbon 0.98-1.1%, manganese 0.25-0.450, 0.025 maximum for each of phosphorous and sulphur, 0.2-0.35% silicon and 1.3-1.6% chromium).
U.S. Pat. No. 5,851,313 relates to a case-hardened stainless steel bearing component and method of manufacturing the bearing.
U.S. Pat. No. 6,203,634 relates to a method for heat-treating through hardened bearing steel components.
U.S. Pat. No. 6,149,743 relates to a method for complete bainite hardening of steel for use in bearings and other load carrying components.